The present invention relates to nuclear magnetic resonance (NMR) angiographic methods for imaging fluid flow in a sample, and, more particularly, to novel NMR angiographic methods in which multiple responses are generated for each excitation of the sample, as for providing medically-significant anatomy images of fluid flow in non-invasive manner.
It is known to provide NMR angiographic data images, indicating the flow of bodily fluids through various bodily passages, for medical diagnostic purposes. Methods for producing such images are described and claimed in U.S. Pat. No. 4,714,081, issued Dec. 22, 1987, assigned to the assignee of the present application and incorporated here in its entirety by reference. While those methods provide true projection images and allow high quality angiograms of arterial and venational structures to be obtained along a selected projection axis and with a selected direction of flow sensitivity in a sample, it is still highly desirable to obtain even further information in the NMR angiograms. For example, projection images along multiple axes of projection are desirable, as an entire flow angiogram can be provided if a plurality of angiograms, each sensitive to orthogonal flow components, can be combined. Further, it may be desirable to improve the signal-to-noise ratio of an angiogram (even along a single axis of projection) to: enhance suppression of motion artifacts; obtain a series of angiograms each sensitive to a different range of flow velocities; and the like. However, with a cardiac-gated NMR angiogram using only one pulse sequence per cardiac cycle, and needing 128 phase-encoding steps in a spin-warp imaging sequence with only one flow direction, the minimum imaging time presently requires about 4 minutes, dependent on heart rate. For two flow directions, requiring twice as many excitation sequences, about 16 minutes may be required for acquisition of data from the patient (sample). It is extremely difficult to maintain the patient in a non-moving state for even these time intervals, much less the additional time intervals required for additional views, etc. Accordingly, methods for providing NMR angiograms with improved features, without requiring substantial additional data acquisition time and without sensitivity to misregistration due to sample movement, are highly desirable.